1. Field of the Invention
The present invention relates to a composite material for a bipolar plate, and more particularly to a composite material having chemical and mechanical stability and good workability, and that can be manufactured at a low cost, and a method of preparing a bipolar plate using the composite material.
2. Description of the Related Art
Fuel cells as electrochemical cells convert energy which is generated by an oxidation reaction of fuel to electrical energy. Fuel cells that are currently commercialized include phosphoric acid fuel cells (PAFC) and molten carbonate fuel cells (MCFC). Polymer electrolyte membrane fuel cells (PEMFC) have also been developed as highly efficient cells. FIG. 1 illustrates a schematic operation view of a PEMFC. A PEMFC comprises a membrane electrode assembly (MEA) 20 including an anode layer 10a and a cathode layer 10b and a polymer electrolyte membrane (PEM) 15 which is interposed between the two electrode layers. The membrane electrode assemblies are laminated using a bipolar plate with fluid flow channels formed thereon. The fuel cell generates electric power by providing fuel and oxidation material into an anode and a cathode respectively, and generating electric power through electrochemical reaction between the anode and the cathode.
As the polymer electrolyte of a PEMFC, a fluorine-containing polymer having an ion-exchange functional group and a group such as sulfonic acid, carbonic acid, phosphoric acid, phosphorous acid, etc. is used. A fluorine-containing polymer electrolyte membrane such as a perfluoro carbon sulfonic acid membrane (Nafion™) which is manufactured by Dupont Company has chemical stability, high ionic conductivity, and good mechanical properties, and thus is generally preferred.
A voltage generated between the anode and the cathode of one fuel cell is generally about 0.7V. Therefore, in order to obtain an appropriate available voltage (10V to 100V), a number of fuel cells need to be laminated to form a stack, and adjacent fuel cells separated by bipolar plates are preferable. The bipolar plate provides an electric connection between the cathode and the anode, and it provides the cathode with a gas flow channel and has strong corrosion resistance and gas impermeability.
The bipolar plate is typically manufactured with a mechanical process using solid graphite or metal, or by molding or a press-process of a graphite-polymer composite material. Such a bipolar plate has a problem of high manufacturing cost and limitary flexibility for stack design. The bipolar plate needs narrow gas channels in order to develop a compact portable fuel cell, but the aforementioned materials have many problems during the manufacturing process. For example, solid graphite is brittle and porous and therefore is easily broken during mechanical processing, and if it is prepared at less than a certain thickness, gas-impermeability is not obtained. Metals such as stainless steel, which is used in place of the graphite, are not preferable because of corrosion.
Therefore, polymer composite materials have recently been developed as a material for bipolar plates since they have high chemical stability, good workability, and low manufacturing cost. For example, a composite material including a fluoropolymer such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), or a phenol resin and conductive carbon has been researched. However, the aforementioned polymer has a problem wherein water is produced as a side product during synthesis of the polymer, and water remains in the resulting polymer.